The formation of protective and decorative oxide coatings on aluminum and aluminum alloys by electrolytically anodizing them is a well-known technology. It has been described in many publications including M. F. Stevenson, Jr., "Anodizing", Surface Engineering, ASM Handbook, Volume 5, ASM International, Materials Park, Ohio, 1994, pp. 482-493 and M. Schwartz, "Deposition from Aqueous Solutions: An Overview", Chapter 10, Handbook of Deposition Technologies for Films and Coatings, Second Edition, R. F. Bunshah, ed., pp. 480-590 (543-545). The most commonly used methods of anodizing use electrolytes of chromic acid (Type I per specification MIL-A-8625), sulfuric acid (Type II), or cold sulfuric acid (Type III). The coatings produced by each are based on alumina, but are usually not pure alumina. For example, the coatings produced using a sulfuric acid electrolyte may contain about 18% aluminum sulfate and 1 to 6% water in addition to alumina. (Unless otherwise noted, all compositional percentage used herein will be in percent by weight.) The alumina itself is commonly a hydrated alumina, 2Al.sub.2 O.sub.3.H.sub.2 O. The oxide coatings are porous and must be sealed to provide adequate corrosion resistance for the aluminum substrate. Hot sealing with pure water may change the alumina coating to Al.sub.2 O.sub.3.H.sub.2 O which presumably increases the volume of the alumina and decreases the porosity. Other sealants, such as dichromates or silicates, tend to form precipitates in the pores, effectively blocking them.
Semiconductor manufacturers extensively rely on anodized aluminum for tooling devices inside of semiconductor processing chambers. These chambers require tooling having both the dielectric and corrosion protection of anodized aluminum. With the use of more aggressive process gases and the increase in processing voltages and temperatures, tooling anodized by current methods is becoming inadequate for semiconductor processing chambers.
Cryogenic treatment of metals is well known, see, for example, R. M. Pillai, et al, "Deep-Cryogenic Treatment of Metals", Tool & Alloy Steels, June 1986, pp. 205-208. Cryogenic treatments include those that lower the temperature of the part to a) about -109.degree. F. (-79.degree. C.) using solid carbon dioxide blocks in an insulated chamber containing the part or carbon dioxide to lower the temperature of an organic liquid in which the part is immersed, b) about -112.degree. F. (-80.degree. C.) in a mechanical refrigerator, or c) about -321.degree. F. (-196.degree. C.) by immersion in liquid nitrogen. One of the cryogenic treatments most commonly done is immersion of the part to be treated in liquid nitrogen (sometimes called deep cryogenic treatment), since, generally speaking, the lower the cryogenic temperature, the more effective the treatment. The effect of the treatment varies from alloy to alloy. One of the most common uses of the treatment results in a more complete transformation of retained austenite to martensite and refined precipitation of carbides in some tool steels. In this case, the resulting change in microstucture and other properties increases resistance to wear.
Aluminum alloys have also been cryogenically treated. Most commercially available alloys fall into the following classification:
2xxx alloys of Al--Cu and Al--Cu--Mg PA1 3xxx alloys of Al--Mn--Cu PA1 5xxx alloys of Al--Mg PA1 6xxx alloys of Al--Mg--Si PA1 7xxx alloys of Al--Zn--Mg--Cu
In the case of typical cast alloys the improvement in properties due to cryogenic treatment is attributed to plastic flow during the cooling and reheating of the alloy which alleviates microstrain within the alloy. In work hardened alloys improvements may be due to more complete transformation of phases, and in alloys that can be precipitation hardened improvements may be due to more complete or widely distributed precipitation. Virtually all of the cryogenic treatment of aluminum alloys to date has been directed at increasing resistance to wear and improved mechanical properties.
It is an object of the present invention to provide superior anodic coatings on aluminum or aluminum alloys.
It is a particular object of the present invention to provide anodized coatings having superior corrosion resistance.
It is also an object of the present invention to provide a process for the production of superior anodized coatings on aluminum or aluminum alloys.
It is a particular object of the present invention to provide a process for producing anodized coatings with superior corrosion resistance.
It is yet a further object of the present invention to provide articles of aluminum or aluminum alloys, such as tooling for semiconductor processing chambers, with superior anodized coatings.
It is yet a further object of the present invention to provide articles of aluminum or aluminum alloys, such as tooling for semiconductor processing chambers, with anodized coatings having superior corrosion resistance.